1. Field of Invention
The application relates to a power control method, and, in particular, to a power control method of an optical disk drive.
2. Related Art
An electric device is sensitive to temperature such that the performance and accurate of the electric device may be affected by surrounding temperature the electric device locates. In order to control the electric device at different temperatures, a proper control module must be designed to modify the affection resulted from the thermal factor.
Referring to FIG. 1, when the temperature of the optical pickup head is at a temperature T1, the output power and the operating current of the optical pickup head are related in a line L1. In other words, the optical pickup head generates the output power at power Pb, Pe, and Pw if the operating current is respectively equal to current Ib, Ie, and Iw.
If the temperature of the optical pickup head rises to a temperature T2, the conventional technique determines if the optical pickup head correctly generates the output power according to a power feedback signal generated by the optical pickup head. In other words, the conventional technique set the power Pe as a base power and determines if the output power of the optical pickup head is equal to the power Pe according to the power feedback signal. At the same time, because the optical pickup head is effected by temperature, the optical pickup head only generates the output power at power Pe′ if it is driven by the operating current equal to the current Ie. Thus the conventional technique adjusts the operating current until the optical pickup head generates the output power at power Pe (as the dotted line A) in the method of close loop feedback control.
However, in order to control the optical pickup head to generate the output power at different powers, the conventional technique computes the operating current driving the optical pickup head to generate other powers according to the current Ie, power Pe, power Pe′ and the other powers. For example, if the optical pickup head is controlled to generate the output power at the power Pw, the conventional technique computes the current Iw′ resulting in the power Pw according to the current Ie, power Pe, and the relationship between the power Pw and the power Pe′. Similarly, the current Ib′ resulting in the power Pb is also computed. Thus, the operating current, which drives the optical pickup head to generate the specific output power, is computed at the temperature T2 in the same way. In other words, the conventional technique computes the operating current resulting in the specific output power at the temperature T2 according to the relationship between the different powers in the line L1 or line L2.
However, when the temperature of the optical pickup head actually reaches the temperature T2, the output power and the operating current of the optical pickup head are not related in the line L2 but a line L3. Regarding to the above example, the operating current computed according to the conventional technique is the current Is′, and the current Is′ drives the optical pickup head to generates the error output power at power Pw′ (as the dotted line B) and power Pb′ (as the dotted line C), which are not correctly controlled at power Pw and Pb. The errors in the above cases are caused by that the conventional technology ignores the threshold current (the intersection of the line L1 and the current axis or the intersection of the line L2 and the current axis). Referring to FIG. 2, the temperature and the threshold current of the optical pickup head are related in an exponential relationship. When the temperature of the optical pickup head raises, the threshold current of the optical pickup head also raises. If the temperature and the threshold current are insufficient, the operating current computed by the conventional technology for obtaining the output power at different temperatures would not have an obvious error. However, if the temperature and the threshold current are sufficient or the output power is to be precisely controlled, the error of the operating current computed by the conventional technique is inevitable. For this reason, the output power of the optical pickup head suffering from temperature changes can not be correctly modified when the operating current of the optical pickup head is only computed based on the power feedback signal and without considering the threshold current of the optical pickup head.
In addition, referring to FIG. 3, the conventional optical pickup head generates the output power in rewriting mode. Because an erasing power Pe is set as a base power, the output power Pout of the optical pickup head is precisely controlled at the erasing power Pe. However, resulting from ignoring the threshold current, the other output power Pout computed based on the base power Pe are not precisely controlled at the bias power Pb and the writing power Pw in the rewriting mode. Furthermore, the output power Pout is away from the bias power Pb and the writing power Pw more (as the dotted line in FIG. 3).
Similarly, referring FIG. 4 to FIG. 6, the conventional optical pickup head generates the output power Pout in a multi pulse writing mode, a castle writing mode, and a block writing mode in the same method described above. In the FIG. 4, because a bias power Pb is set as a base power, the output power Pout is precisely controlled at the bias power Pb but not at the writing power Pw (as the dotted line) In the FIG. 5 and FIG. 6, because a bias power Pb and a writing power Pw can set as a base power in the castle writing mode and the block writing mode, the output power Pout is precisely controlled at the bias power Pb and the writing power Pw, but not at a writing power Ps and a cooling power Pc (as the dotted line). No matter the optical pickup head is in what access mode, ignoring the threshold current causes that the output power Pout is not precisely controlled. Therefore, the optical pickup head does not correctly access the optical disk, and, even more, the lifetime of the optical pickup head and the endurance of the optical disk drive are reduced.
It is therefore a subject of the invention to provide a power control method for an optical disk drive, which considers a temperature of an optical pickup head and an accessing mode of an optical disk to control the optical pickup head to generate the output power correctly at different temperature and in distinct accessing mode.